This invention pertains to antenna systems for azimuth-or-bearing indicators.
There are many instances where it is necessary to know at a site the bearing angle of a remote source of microwave power. For instance, ships quite often need to know the position of other ships in fog or at night, particularly, when the other ships are hunting targets by means of search radar. Similar situations arise between airplanes and radar guided missles. Existing systems performing a similar function consist of a circular array of elements (usually cavity-backed spirals) with carefully matched radiation patterns, amplitude tracking log-video detectors, and the angle of arrival is interpolated by comparing the relative amplitudes of the receive channels. The bearing resolution of this type of discriminator is poor, especially considering frequency response which depends on precise amplitude tracking of the elements and the log-video receivers versus frequency.
Other such bearing angle indicators required rotating antennas wherein the instantaneous angular position of the antenna was used in determining the bearing of a microwave power source. If should be apparent that rotating antennas are not only unreliable but because of their mechanical configuration add complexity, weight and bulk to the system. In addition the rotating antenna is also quite slow, typically of the order of 1 revolution per second, and covers only a very small angle at any instant of time.
There have been proposals for instantaneous bearing monitors using fixed (non-rotating) antennas comprising four radiators connected via a 4-input, 4-output Butler matrix to a two-input phase discriminator whose output feeds a cathode ray tube display. Such systems can only give coarse bearing indications because of inherent errors in the system. In NRL Report 8005 entitled "Ambiguity-Resistant Three- and Four-Channel Interferometers" by Robert Goodwin there are proposals for devices using more than four radiators. However, such devices are concerned with linear arrays which cannot scan more than 180.degree. of azimuth range.